1. Field of the Invention
The present invention relates to a traveling-wave fed array antenna, and a radar apparatus using the array antenna.
2. Description of Related Art
There is known a vehicle-mounted radar apparatus which scans ahead of a vehicle in the lateral direction (horizontal direction) of the vehicle with a radar beam to detect an obstacle or a preceding vehicle present on the traveling lane of the vehicle.
Also, as an antenna for use in such a radar apparatus, there is known a traveling-wave fed array antenna 101 having a structure shown in FIG. 8A in which a plurality of radiating elements 103 are arranged in a row, and connected in series through a feed line 105, the feed line 105 being terminated at one end thereof with a resistor to prevent a reflected wave from occurring, and being fed at the other end thereof.
Such a traveling-wave fed array antenna 101 is mounted on a vehicle plurally along the lateral direction to enable detection in a lateral plane, such that the arranging direction of the radiating elements 103 is along the vertical direction.
Incidentally, the beam direction of the traveling-wave fed array antenna 101 varies with the variation of the frequency of the traveling wave fed thereto. For example, as shown in FIG. 8B, when the arranging interval (feed line interval) D between the succeeding radiating elements 103 is equal to the on-line frequency of the fed signal (when the on-line frequency is f1 in FIG. 8B), since all the radiating elements 103 radiate radar waves having the same phase, the direction of the beam transmitted from the traveling-wave fed array antenna 101 points to the front direction (the tilt angle =0) of the radiating plane on which the radiating elements 103 are disposed. On the other hand, when the arranging interval D is different from the on-line frequency of the fed signal, since the radiating elements 103 radiate radar waves having different phases successively increasing by a constant value α along the arranging order of the radiating elements 103, the direction of the beam transmitted from the traveling-wave fed array antenna 101 has an inclination depending on the constant value α to the front direction (the tilt angle=0) of the radiating plane.
Accordingly, various methods to keep the tilt angle unchanged when the frequency of the fed signal is changed are proposed. For example, refer to Japanese Patent Application Laid-open No. 08-097620, or No. 2006-279525. Incidentally, when a radar apparatus is mounted on a vehicle, the direction, especially the elevation tilt angle of the radar beam has to be adjusted.
Such tilt angle adjustment can be carried out by manual work using a screw. It is also known to carry out the tilt angle adjustment by performing electronic signal processing such as DBF (Digital Beamforming) or MUSIC (Multiple Signal Classification). Further, it is also known to perform beam scanning in the elevation direction by use of a specific hardware device such as a dielectric lens, a Rotman lens or a Butler matrix, and set the beam transmission angle to a desired elevation tilt angle. However, performing such electronic signal processing or using such a specific hardware device causes the circuit scale and signal processing amount of the radar apparatus to increase.
Accordingly, it is proposed to electrically adjust the tilt angle making positive use of the fact that the tilt angle varies with the variation of the frequency of a fed signal. For example, refer to Japanese Patent Application Laid-open No. 2006-64628.
However, since the frequency band of a vehicle-mounted radar apparatus is limited to the narrow range (76 GHz to 77 GHz), the tilt angle can be changed only by approximately 2° at most (approximately ±1°) when its radiating elements are arranged at intervals of one wavelength of a fed signal) even if the frequency of the fed signal is varied to a maximum extent possible within the above range, which is insufficient to adjust the tilt angle sufficiently.